The present invention relates to a low pulsation pump device, and more particularly to a low pulsation pump device which is capable of delivering liquid with low pulsations and is thus suitable for use in liquid chromatography, ion chromatography, or GPC (Gel Permeation Chromatography).
An example of a conventional low pulsation pump is a computer-controlled dual pump in which a pulse motor is provided for each of two plungers so that the two plungers essentially operate as two independent pumps. The control performed to reduce pulsations in the liquid delivered by this pump is merely an adjustment of the phase difference between the two pumps, and is not essentially different from control in which the phase difference is mechanically adjusted so as to be fixed. More specifically, if, for instance, the phase is adjusted in such a way that a pulsation is minimal in a portion of the period in which the end point of the discharge of one of the pumps overlaps the start point of the discharge of the other pump, no adjustment is provided with respect to a pulsation in a period portion in which the start point of the discharge of the first-mentioned pump overlaps the end point of the discharge of the other pump. As a result, the reduction in pulsations is imperfect if the pumps are not operating under exactly the same mechanical conditions.
Japanese Patent Laid-Open Publication No. 128678/1980 and Japanese Patent Laid-Open Publication No. 98572/1981 disclose conventional plunger pump devices The former proposal discloses a structure in which a single cam drives two pumps. Since the discharge pressure of the pumps is detected in a real-time manner to determine the start point and end point of each of high speed driving regions of the pumps, ripples cannot be completely removed because of the time lag in the feedback loop. The latter proposal discloses two plunger pumps driven by a single cam in such a way that a predetermined discharge amount is obtained by combining the liquid flows from the two pumps. The latter proposal also teaches estimation, on the basis of data on the detected rotational position of the cam, a period of time which is required until the predetermined flow rate recovers, and to change the rotational speed of a pulse motor during the particular period which has thus been estimated.
Since each of these conventional plunger pump devices includes two pumps incorporated as one unit, it has a complicated structure. In addition, since the optimization control of the pump device is nothing more than a phase adjustment between two pumps, the resulting reduction in pulsations will often be insufficient.
U.S. Pat. No. 3,855,129 discloses another pump device. However, since this pump device is adapted to control pressure fluctuations by detecting the discharge pressure of the pump in a real-time manner, pulsations can be reduced only imperfectly because of the inevitable time lag.